ARROW-2883: [C++] Fix Clang warnings in code built with -DARROW_GPU=ON

[wesm]

I also opened issues ARROW-2887, ARROW-2888 which surfaced on handling these warnings

[codecov-io]

Codecov Report

Merging #2298 into master will increase coverage by 2.39%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master    #2298      +/-   ##
==========================================
+ Coverage   84.29%   86.69%   +2.39%     
==========================================
  Files         292      236      -56     
  Lines       44559    41820    -2739     
==========================================
- Hits        37562    36255    -1307     
+ Misses       6966     5565    -1401     
+ Partials       31        0      -31

Impacted Files	Coverage Δ
cpp/src/plasma/client.cc	83.86% <ø> (ø)	⬆️
cpp/src/plasma/test/client_tests.cc	100% <ø> (ø)	⬆️
cpp/src/plasma/protocol.cc	98.66% <ø> (ø)	⬆️
cpp/src/plasma/store.cc	88.88% <92.3%> (+0.04%)	⬆️
go/arrow/memory/go_allocator.go
go/arrow/array/numeric.gen.go
go/arrow/memory/checked_allocator.go
rust/src/lib.rs
go/arrow/array/booleanbuilder.go
go/arrow/type_traits_numeric.gen.go
... and 50 more

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[wesm]

+1